Clones for human prothymosin alpha have been identified in cDNA libraries from staphylococcal enterotoxin A-stimulated normal lymphocytes and from simian virus 40-transformed human fibroblasts. The 1198-base-pair fibroblast clone has been sequenced. The encoded protein is highly acidic and shares greater than 90% sequence homology with rat prothymosin alpha. The peptide "hormone" thymosin alpha 1 appears at positions 2-29 of the prothymosin alpha amino acid sequence. These is no signal peptide. Prothymosin alpha is inducible and enjoys broad tissue specificity. In two systems, the mitogen stimulated resting lymphocyte and the serum deprived NIH 3T3 cell upon serum restitution, an increase in the level of prothymosin alpha mRNA accompanies cell growth. There are at least four genes, three of which have been isolated from cosmid libraries. There is no evidence that prothymosin alpha serves as a precursor for secreted thymic peptides or that its function specifically involves modulation of the immune system. Rather, prothymosin alpha appears to play a role in cell proliferation.